Received 11 June 2015. Revised 5 August 2016. Accepted 28 August 2016. Available online 3 September 2016.

Highlights

MOR and MOE are appropriate indicators to quantify a WPC’s ageing.

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Accelerated weathering using xenon-arc light most degrades a WPC’s strength.

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A PE-based matrix has a favourable effect on a WPC’s durability.

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Strength losses are transferable to a WPC cladding design value.

Abstract

The successful development of durable wood-plastic composite (WPC) cladding is a challenge for the industry. Due to the organic fibres in the compound it is though doubtable if WPC performs satisfyingly in a façade application which is expected to last 50 years. This paper provides insights from a study to find out how ageing of WPC cladding can be assessed by relevant norms and which material components best support durability aspects. Based on these, a meta-analysis using 44 empirically generated data from 12 papers in the field of accelerated weathering, thermohygric conditioning and fungal decay of WPCs was conducted. It was found that high values of modulus of rupture (MOR) and Young’s modulus (MOE) are considered key to an engineering design of WPC façades. Both parameters are appropriate indicators to describe the ageing of WPC cladding. Weathering of WPC decreases MOE more than MOR. Exposure to UV-radiation is more harmful than frost-thaw cycling. In order to minimize a loss in value in MOR and MOE, the use pf polyethylene in extruded panels with large-sized hardwood fibres under low content is recommended. Hence, these basic findings add higher value to a product development.

Keywords

Wall cladding

Wood-plastic composites

Product development

Material ageing

Meta-analysis

1 Introduction

In recent years deciders in building products and materials are more and more driven by durability and sustainability aspects. This is not only because norms and codes in the construction industry steadily take these criteria into account. In times of climate change, scarce fossil resources and increasing concerns about the vulnerability of the earth, architects and engineers feel more than ever morally responsible for the consequences of their decisions. But it is also up to the building product manufacturers to provide alternatives to conventional and less ecological products. The underlying philosophy is that a building material becomes more sustainable by increasing the share of biological components [1], [2], [3], [4] and [5]. However, the case is more complex than it may first appear. The more hydrophilic organic ingredients are used in cladding products, the higher is water uptake and the less is the material’s durability when impacts like frost and fungi occur [6] and [7]. Hence, manufacturers are faced this challenge when developing bio-based building products which are expected to last several decades in outdoor applications.

One example for bio-based materials is wood-plastic composites (WPC) which consists of wood fibres embedded in a petrochemical plastics matrix. So far the main applications are decking and cladding [8]. There is an emerging body of literature revealing the opportunities of WPCs in the building scope [9] and [10]. Basically, WPC uses up to 80% plant fibres and only 20% of fossil-based thermoplastics such as polypropylene (PP), polyethylene (PE) and polyvinylchloride (PVC) [11]. The high fibre share obviously relives the pressure on scarce resources. Furthermore, WPC supports the use of both recycled plastics and wood [2]. Even more, the compound itself can be recycled which follows the principle of cradle-to-cradle and which postpones the question of disposal to the future [12], [13] and [14]. And finally, the production of WPC and its recycling is comparatively less energy consuming compared to conventional building materials such as metal or cementitious products. WPC is compounded by 180 °C whereas cement is sintered by 1200 °C. Taking a broader view, WPC provides interesting future aspects. The remaining 20% of petrochemical polymers can be substituted by bio-plastics such as polylactic acid (PLA) or polyhydroxyalkanoates (PHA) [15] and [16]. This turns current WPC into a Green-Composite which solely uses renewable resources. Products made from it are theoretically compostable. Hence, WPC offers new opportunities for the construction industry and specifically architects seeking a Green-Building Certificate will appreciate such a Green-Composite Façade (GCF) [17] and [18]. In this regard, WPC is viewed as being a transitional technology on the way to future green-composite building products [19].

The production volume of WPC worldwide was 2.43 million tons in 2012. Europe accounted for 260,000 tons and further increases to 450,000 tons are forecasted for the year 2020. Decking made 67% whereas cladding and fencing reached only 6% of the total amount in Europe [10] and [20]. Obviously besides decking, fencing could not yet emerge as market innovation. In addition to decking, which competes against less ecological tropical hardwood or maintenance-intensive domestic softwood, WPC cladding has to match materials which by experience performs satisfyingly over 5 decades. This life-time is determined by norms, such as EN 1990 [21] and EN 1991 [22], which provide 50-year wind loads as basis for the proof of the cladding’s structural fitness. While to date, WPC cladding could not yet demonstrate if it indeed lasts 50 years in a façade application. It is undoubtable that each material will suffer from strength decreases provoked by ageing effects. The key is to find out about the expected strength loss of a WPC cladding even before it is installed at a building. Based on the literature reviewed, there is not yet any national or European approval for WPC cladding [23]. Such documents usually provide design values according to which a façade planner could demonstrate the 50-year structural resistance of the selected product in a target building under normative wind loads. However, some papers in the field of durability testing of WPC reported about strength decreases which at least give orientation in the assessment of a reasonable design value for the proof of structural fitness [24]. Based on the author’s experience from the WPC industry, manufacturers are faced a lack of information about how to assess this long-term material degradation for their product. Therefore, the following questions are of paramount interest: (1) Which are the relevant norms to take into account when empirically investigating WPCs durability; (2) which are according these norms the most relevant impacts on WPC cladding during their life-time which significantly provoke ageing; (3) how can ageing become quantified from laboratory tests and (4) which aspects in WPC cladding product development have highest potential to optimize the product’s long-term performance?

This paper reports about how these basic questions were successfully answered by research in legal regulations and material technology for WPC.

2 Normative regulations in the scope of cladding and WPC

2.1 Construction Products Regulation EU 305/2011

The European Regulation (EU) No 305 [25], known as Construction Products Regulation (CPR), entered into force as from 2011 and it forms the basis for the development of building products. According to the CRP, a cladding is a building product if its purpose it to rest permanently in a building or a part of it and if its properties moderate the building performance. A WPC cladding undoubtedly meets this criterion laid down in Article 2.1. Furthermore, the CPR defines seven basic prerequisites to be considered in a product development: (1) Mechanical resistance and stability; (2) Safety in case of fire; (3) Hygiene, health and the environment; (4) Safety in use; (5) Protection against noise; (6) Energy economy and heat retention; (7) Sustainable use of natural resources. Special attention is paid to safety aspects which are set down by the following: “Buildings and building parts have to be erected in the way that they under no circumstances represent a danger for public safety, health and livelihood.” A building part complies with these demands if it is stable. In terms of façades, the stability of a cladding product is regulated by norms, such as a harmonized European norm (EN) or a national norm. If such norms are lacking for a particular material, there is hence a possibility to regulate a cladding by building product approvals. This could either be a European Technical Assessment (ETB) in case of an EU-wide regulation or a national approval for a country-wise employment. Although there is no strict rule whether in case of the absence of an EN the producer has a choice between regulating a product as per state of the art or per ETB it is commonly assumed that the former can only be taken into account if the particular product is long-standing established in the building scope which in terms of WPC as a novel product cannot be the case. A study on how to regulate WPC cladding therefore should focus on norms and guidelines for the execution of product kit approvals with regard to a façade application.

All further considerations in this paper consequently follow the requirements set out by a WPC-related harmonized norm, as best case. Such a document must contain further information about which product properties need to be declared to the market by the manufacturer.

2.2 Harmonized European WPC norm: EN 15534

As per April 2014 the European Norm (EN) 15534 [26] was introduced into the EU-member countries. Developed by the Mirror Committee CEN/TC 249/WG 13: wood-polymer composites (WPC), this norm specifies compounds which are made from natural fibres mixed with polymeric plastics. The used fibres are derived from plants and as such hemp, sisal, coconut, cotton, kenaf, jute, abaca, banana, leaf fibres, bamboo, rice, wheat straw etc. Polymers are expected to be virgin or recycled. The following Table 1 provides an overview about the norm series 15534 and Part 1 and 5 were applied in this study.

Table 1. European Norm 15534 series for natural fibre-reinforced composites.

Norm

Content

EN 15534-1: 2014

Composites made from cellulose-based materials and thermoplastics (usually called wood-polymer composites (WPC) or natural fibre composites (NFC)) – Part 1: Test methods for characterisation of compounds and products

EN 15534-4: 2014

Composites made from cellulose-based materials and thermoplastics (usually called wood-polymer composites (WPC) or natural fibre composites (NFC)) – Part 4: Specifications for decking profiles and tiles

EN 15534-5: 2014

Composites made from cellulose-based materials and thermoplastics (usually called wood-polymer composites (WPC) or natural fibre composites (NFC)) – Part 5: Specifications for cladding profiles and tiles

EN 15534-6: 2015

Composites made from cellulose-based materials and thermoplastics (usually called wood-polymer composites (WPC) or natural fibre composites (NFC)) – Part 6: Specifications for fencing profiles and systems

EN 15534 Part 1 contains information about which material properties need to be elaborated for WPC compounds and which synthesized test methods should be applied. EN 15534 in general represents a basic material norm rather than a product norm because it also contains criteria which are not relevant to cladding. Table 2summarizes key material properties and threshold values given by this norm. Façade-oriented attributes which should be considered in a development process are marked in bold.

EN 15534 Part 5 is viewed as being at least application-oriented because it selects those material criteria from Part 1 which are applicable to the use of a WPC compound in façades. It is important to know that Part 5 explicitly excludes its application for cladding kits, comprising WPC panels, sub-construction and fasteners (EN 15534-5, Chapter 1). However, in addition to Part 1, this norm quantifies minimum values which ensure that the cladding product at least performs in a particular way which fits to its application, such as dimensional accuracy or maximal thermal linear expansion.

The list of measurable material attributes for WPC is long and they can be summarized by the following categories:

a)

Stability: Tension and flexural stresses are indicators for the material resistance against external load impacts. Stresses at breaking, such as modulus of rupture (MOR) as well as Young’s modulus of elasticity (MOE), are considered key traits in this field. By experience, the higher is MOR and MOE of a material the better it fits to a façade application. It should be noted that both values are characteristic in nature because they emerged from test series which results are statistically treated on a 5%-fractile base. Furthermore, the stability of façade coverings is also subject to the resistance of the fixation of single panels.

b)

Durability: This aspect deals with strength decreases due to external impacts over the product’s life-time. The effects are simulated by both artificial and natural ageing tests. In the former, samples are pre-treated under freeze-thaw cycling and under impacts of basidiomycetes. From both the loss of bending strength is measured. Natural ageing is investigated by the exposure of samples for one year to weathering conditions equal to South France. Here as well the loss of strength is measured afterwards. Category “Durability” therefore provides the basic tests to assess the material weakening due to (1) freeze-thaw cycling, (2) basidiomycetes and (3) UV-exposure.

c)

Serviceability: Cladding is expected to keep its form and appearance at least for a considerable life-time and under changes which approach rather slightly. A constant geometrical appearance is ensured by low linear thermal expansions of the WPC panels, moderate creeping and little swelling due to water impacts. Discoloration is evaluated by xenon-light pre-treatment of samples and subsequent peeling tests of surface coverings.

d)

Safety: Here, the fire resistance of wall panels is viewed as being most important in façade applications.

e)

Other criteria: This category contains further characteristic traits and as such termite resistance, if WPC façades are erected in Mediterranean regions, the bulk density which allows the derivation of shear loads or the impact resistance which helps assessing the suitability of the cladding for hard-body attacks which for instance occur in hailstorms.

According to Part 5 of EN 15534, the manufacturer must execute initial tests and declare a particular list of selected compulsory and voluntary product traits. As a consequence, most product attributes are elaborated only once unless future changes in material formulation or production process would make a difference to former results. It is therefore plausible that findings from these tests should also serve a strength proof for a WPC façade carried out by engineers as demanded by EN 1990 [21] and 1991 [22]. However, it is important to note that all attributes are characteristic in nature which means that they represent a real reaction of the material after a natural or artificial pre-treatment. Hence, test results don’t represent the future conditions under which the product must perform in its final destination.

Anyhow, the literature pertaining to WPC’s durability sees in the existing WPC norm a measurement of limited suitability for quantifying the loss in material strength for the practice [27] and [28]. As a basic conclusion, EN 15534 describes durability tests which are not necessarily appropriate to assess the real product behaviour in practice but which are obligatory for manufacturers. It is therefore from highest interest to find a way how to assess WPC’s real ageing behaviour from these initial tests which are anyway time-intensive and costly. The transfer of characteristic test results, such as MOR, into the practice is usually done by a simple conversion factor. However, there is no normative basis according to which such a factor can be elaborated for WPC [24]. Its development demands a deep knowledge in both the application and material technology.

2.3 Guideline for the execution of a building product approval: ETAG 034

ETAGs are European Technical Approval Guidelines which describe how approval documents should be carried out in case a harmonized norm is lacking. The ETAG 034 [29] was launched in July 2011 by the EOTA (European Organization for Technical Assessment) and it contains information about the deliverable initial tests, the conformity attestation and CE-marking of ventilated façade product kits. As already stated, a WPC cladding material is described by the EN 15534 which explicitly excludes its application for cladding kit product systems. As the study on hand attempts to regulate a WPC cladding for its application, it is essential to consider aspects from this ETAG even if the manufacturer doesn’t seek an ETB. In addition to the EN, this guideline integrates aspects of performance behaviour due to interaction of single product components, such as fasteners, the coverings and the sub-construction. It is relevant to note that the application of this ETAG should be done in combination with material-related norms. Although the ETAG only covers a selected number of materials, such as fibre cement, pure plastics, ceramics, timber etc., it seems plausible to apply this guideline also to other materials for which an EN material norm already exists, like WPC.

The additional aspects under which cladding kits are investigated according to this ETAG can be summarized as follows:

(1)

Water tightness (protection against driving rain), chapter 5.3.1: The water tightness of a mock-up of 1.20 m × 2.40 m is tested under artificial weathering using simulated wind-driven rain by 600 Pa. The cladding system is viewed as being water tight if on the backside no discernible water drops are visible.

(2)

Wind load resistance, chapter 5.4.1.1: Cyclic wind suction is applied to a test surface of 1.5 m2 with step-wise increase from 300 Pa up to cladding failure. The deformations and the failure loads are measured. This test is mostly executed in a suction chamber which makes this approach to a costly undertaking. However, the ETAG 034 also proposes using a foil bag at the rear side of the cladding and which becomes inflated by compressed air until failure.

(3)

Mechanical resistance of fasteners, chapter 6.4.2: The pull-out resistance of fasteners, as screws, rivets or metal clips, through the WPC material can be elaborated using testing machines for tensile or bending strength. The rupture load and the deformations are measured from which the stiffness of the fixation points can be derived. Such insights are often applied in structural analysis by FEM simulations.

(4)

Durability, thermohygric product behaviour, chapter 5.4.5: In addition to freeze-thaw cycling according to EN 15534, the ETAG demands a comparable test procedure applied to a 6 m2 cladding surface. After artificial weathering, comprising also a spraying with water, the effects on the surface, such as peeling, cracking, erosion etc. is visually assessed and documented.

In addition to EN 15534, the ETAG 034 rather focuses on the performance of a complete façade section. However, neither the ETAG 034 nor the EN 15534 provide information on how the established durability-related test results can be condensed to a conversion factor which describes the material ageing. Such a factor would give orientation in a product development which primary aims to create highly durable WPC claddings.

2.4 Concept for the derivation of a design value from characteristic test results: EN 1990

EUROCODE EN 1990: Basis of structural design [21] is a base norm which regulates how structural calculations in the building scope should be carried out. Whenever a building element or a material is subject of strength proof, this norm specifies the treatment of material-related properties which were derived from performance tests. This is necessary because results from such tests solely show actual features of a particular series of test samples. It is doubtable if they could serve as reference for all imaginable cases under which WPC is used in practice. Therefore, results from such tests must be transferred from a characteristic level, representing the virgin state, to a design level serving for the code of practice. Of course, a material resistance after such a treatment must become inferior which takes into account all distortion effects within the test procedure, size effects resulting from small test samples and future large-scale applications, further deviations from the test conditions as temperature, humidity and specifically ageing effects within the application-related time frame. EN 1990, chapter 6.3.3 comprises such aspects by a single factor, which is:

equation1

To additionally compensate uncertainties in the previously determined conversion factor and for corrections pertaining to deviations between the material and the product made from it, EN 1990 recommends applying a partial safety factor γM.

Taking both factors into account, the resulting engineering design value Rd derived from material tests is as follows:

equation2

where Xk is the material property (EN 1990, 4.2) given as the 5%-fractile value from a test series using virgin specimen.

Hence, the factor ɳ takes into account the difference between the bias of a virgin test sample and the same material in a future application close to its end-of live. Therefore it is at first necessary to determine the particular influences which most moderate this discrepancy. Based on the previous findings from the literature review in norms and regulations it seems plausible that (1) exposure to natural weathering, (2) fungal decay and (3) thermohygric conditions like freeze-thaw cycling have the highest impact on WPC ageing to be covered by ɳ in Eq. (2). These tests measure the weakening due to particular pre-treatments which most widely and realistically catch the natural environment of a WPC cladding façade over its life-time.

3 Methodology

Introductory studies so far revealed three initial tests which are compulsive for WPC cladding manufacturers when regulating their product according EN 15534. All three tests use the material’s bending strength as dependent variable to describe the ageing effect. Natural weathering should take place in Europe’s Mediterranean region where the annual radiation dose is supposed to be 6,6 GJ/m2[26]. In addition to this, accelerated artificial weathering tests with laboratory light sources are proposed as alternative. This approach is far less time intensive and lasts 12.5–88.3 d depending on the norm and light source. Basic norms in this field are figured out in Table 2. WPC’s reaction to water is measured by EN 321 [30] and it uses cycles of water soaking for 72 h, frost periods for 24 h and drying phases for 72 h. Mostly 3 cycles are passed where the first one has an extended phase for water immersion taking 28 d. This test in total takes 49 d. And finally, strength decrease due to fungal decay is measured according to ENV 12038:2002 [31] using Coniophora puteana (wet-rot), Gloeophyllum trabeum (brown-rot) und Coriolus versicolor (white-rot) in the specimen pre-treatment. The strength loss of incubated specimens is measured by bending tests of pre-dried and wet samples. By this comparison the effect of fibre deterioration becomes discernible because the water uptake alone also leads to strength decreases.

So far the preparative investigations on relevant regulations could satisfyingly answer the basic questions about which norms and tests could serve the assessment of WPC ageing behaviour. They build a theoretical framework according to which the decrease of the material’s virgin strength under artificial climates becomes quantifiable. It can be argued that this framework is not appropriate to simulate a cladding’s complete life-time because the number of frost-thaw cycles or the duration of radiation impacts as well as the intensity of exposure obviously cover only a share of the expected dose in a cladding’s life. However, for a development of a WPC cladding’s design value according to Eq. (2) it is more interesting to know which criteria have the highest effect on the ageing. WPC is a complex material and its performance mostly depends on the composite’s ingredients and the production method which by today is dominated by extrusion and injection moulding. Current WPCs differ in the types and shares of fibres, plastics and additives used in the formulation. The production process itself is influenced by the composite’s components and the end use of the product. The high number of possible compositions therefore makes it hard to predict a WPC’s characteristics prior to development. Extensive previous literature review has witnessed that research in WPC is predominately settled in basic material investigations which employ single- to bi-variate analysis using only one kind of the three described impacts [19].

The study on hand therefore investigated relevant papers in the field of the three previously described durability tests. Results were assessed by a meta-analysis which tried to reveal how basic factors in WPC formulations and production processes affect the durability of current WPCs. To make the empiric results comparable among the papers, only those using bending strength MOR ad Young’s modulus (MOE) as dependent variables under similar conditioning methods were selected. Following these requirements, in total 12 papers were examined, where 5 of them report about resistance to artificial weathering, 3 about fungal decay and another 4 about thermohygric impacts. Additionally, 9 papers were selected which examined further dependent variables such as water content or weight loss which by trend indicate the change of MOR and MOE as well. In terms of UV-impacts, the number of papers reporting about natural weathering of WPC was too small which is why the study focussed solely on artificial accelerated weathering trials. For the meta-analysis in total 44 variables could be derived from the 12 papers. Statistical calculations by SPSS Statistics and a ranking of all dependent variables tried to reveal which specimen showed minimal strength loss and therefore best support a façade strength calculation using Rd as per Eq. (2) and MOE for serviceability proofs.

4 Case study results and discussions

The following sections summarize the results from the meta-analysis for each type of conditioning method (1) accelerated weathering, (2) fungal decay and (3) thermohygric impacts. Table 3, Table 5 and Table 7 provide an overview about the referenced papers, the compound formulation used, the independent variable, the applied production method of specimen, the conditioning method and a specimen code connected to the dependent variable MOR and MOE. The Table 4, Table 6 and Table 8 show the absolute results for MOR and MOE and their losses in value. Finally, the results from the statistical analysis are depicted in Fig. 1, Fig. 2 and Fig. 3.

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